Many water-soluble non-ionic compounds are produced by synthesis or fermentation along with a certain amount of salt or ionic impurities. One of the conditions necessary to a successful purification is the availability of an efficient tool for the separation of the product from salts or ionic impurities.
In the last two decades, the following technologies have become available to separate non-ionic compounds from salts or ions, which have been produced along the syntheses or during the purification process:
nano- or ultra-filtration (Bungay P. M. et al., "Synthetic Membranes", Science Engineering Application, D. Reidel, C181, 1986; Applegate L. E., "Membrane Separation Process", Chem. Eng., 63-89, 1984); PA1 electro-dialysis (Bungay P. M. et al., "Synthetic Membranes", Science Engineering Application, D. Reidel, C181, 1986); PA1 ion exchange in separate beds or in mixed bed (B. Coulter, Ultrapure Water, November 87, Continuous Deionization, CM, Mag., 19, 29, 1993); PA1 Continuous Deionization (CDI) (Acconazzo, Mauro A., Fluid/Part. Sep. J., 7, 29M, 1994).
The first two methods offer a very practical and economical tool for eliminating large amounts of salts from relatively high molecular weight substances, but are not so suited for reaching a very high reduction ratio in salt concentrations and a very low final salt concentration. In fact, for ultra- and nanofiltration the reduction ratio, which is defined as the ratio between the salt concentration before and after the treatment, depends on diafiltration volume and time by a negative exponential function. From a theoretical point of view, this fact demonstrates that high reduction ratios are very expensive in terms of volumes to be treated and plant dimensions.
In the case of electro-dialysis, reaching very low salinity values is slow and inefficient due to low electrical conductivity of the solution.
Furthermore, neither nano- (or ultra-)filtration nor electro-dialysis are capable of removing relatively large organic ions, which are often present as synthesis by-products, and are often responsible of unwanted discolouring of the product solutions.
For the reasons above explained, final desalting, that is desalting to very low ion concentration, for example below 10.sup.-4, preferably 10.sup.-5 or 10.sup.-6 mol/L, is therefore always performed by ion exchange or by CDI.
A state-of-the-art ion exchange unit for final desalting consists of 2 or 3 columns arranged in series, wherein the first one must contain a strongly acidic cation exchanger in H.sup.+ form, optionally together with a weakly acidic cation exchanger; and the second one contains an anion exchanger, which can be either a weakly basic anion exchanger in free base form or a strongly basic one in OH.sup.- form.
The equipment may contain a third column arranged in series, which is again filled with a cation exchanger.
In a device of this type, the strongly acidic cation exchanger contained in the first column is essential to remove neutral salts such as NaCl or Na.sub.2 SO.sub.4 : in fact, these salts are not affected by the treatment with weakly acidic exchangers, since a weakly acidic exchanger can not displace strong acids from their salts, for example HCl from NaCl (it has no salt-splitting capacity).
As a consequence, a conventional separate beds-ion exchange unit can not deionize compounds that are not stable when in contact with the strongly acidic ion exchanger itself.
Mixed-bed units can not solve the problem, as weakly acidic cation exchangers are not applicable to mixed beds. In fact, the separation between anion and cation exchangers, which is necessary to regenerate the mixed bed, can be performed only if there is a high difference in the densities of the anion and cation exchangers, while weakly acidic cation exchanger have densities which are very near to the anion exchangers' ones.
Finally, also CDI is based on a mixed bed in which cationic component is strongly acidic.
In conclusion, in the state of the art, all the technologies available for final desalting, that is all the technologies that allow to reduce neutral salt concentration to below 10.sup.-4 mol/L or to reduce neutral salt concentration of a factor grater than 100, involve the use of strongly acidic cation exchanger. As a consequence, final desalting of compounds that are not stable in contact with the strongly acidic sites of the exchanger or that can be protonated and fixed by the strongly acidic sites of the exchanger is difficult and gives normally poor results in terms of residual salts content and yield.
Another difficult case to treat with presently available desalting technologies is the one in which the product itself is stable to the contact with the strongly acidic cation exchanger, but some of the impurities react with the strongly acidic cation exchanger to give other impurities, which can no more be eliminated from the ion exchangers. In this case, the lack of a final desalting technology that does not require the contact of the solution to be desalted with a strongly acidic cation exchanger makes the removal of these reacting impurities difficult, if not impossible.